JP3802843B2 - Optical fiber manufacturing method - Google Patents

Description

[0001]
BACKGROUND OF THE INVENTION
The present invention, among the optical fiber used in the optical communication field, a method of manufacturing the fiber optic to allow further large capacity communication.
[0002]
[Prior art]
An optical fiber that enables high-capacity and high-speed communication is indispensable for constructing an optical communication network. However, in recent and future optical communication networks, optical signals have been increased in speed and information. Accordingly, an optical fiber having a larger capacity has been demanded. At present, an optical fiber called a so-called photonic crystal optical fiber has been attracting attention as a new optical fiber that satisfies this demand.
[0003]
The photonic crystal fiber is an optical fiber using a photonic crystal (PC) having a uniform two-dimensional periodic structure in the longitudinal direction of the fiber as a clad covering the core, and a region corresponding to the clad. Is provided with a photonic band gap (PBG) and the light wave is confined in the core by Bragg reflection.
[0004]
The photonic crystal optical fiber proposed so far is, for example, a hole that is continuous in the longitudinal direction of the fiber in the cladding, such as a holey fiber (HF), and the effective refraction in that region. This is achieved by reducing the rate. This holey fiber has characteristics that cannot be achieved with ordinary optical fiber, such as ultra-wideband single-mode transmission region, large effective core area, high refractive index difference (High- △), and large structural dispersion due to the hole design in the cladding. Is feasible.
[0005]
For example, as shown in FIG. 3, such a photonic crystal optical fiber is obtained by cutting a small-diameter quartz rod a having an outer diameter of about 500 μm and a small-diameter quartz tube b having an inner diameter of about 300 μm to a length of about 300 mm. The small diameter quartz rod a is bundled so as to be surrounded by several hundred small diameter quartz tubes b, b... And the bundle c is inserted into a quartz jacket tube d having an inner diameter of 10 to 15 mm and an outer diameter of about 25 mm. After the preform e is formed, the preform e is fused and integrated with the bundle c of the fine quartz rod a and the fine quartz tubes b, b... And the quartz jacket tube d by a normal optical fiber drawing process. In this way, it is obtained by drawing to a predetermined fiber diameter of 100 to 150 μm.
[0006]
And in such a photonic crystal optical fiber, as shown in FIG. 3, the axial center part which consists of the said small diameter quartz rod a becomes a core area | region which propagates light, and the surrounding small diameter quartz A portion made of the tubes b, b... Becomes an inner clad layer having a large number of holes, and a solid portion made of the surrounding quartz jacket tube 5 becomes an outer clad layer, and most of the light waves propagating through the core region are contained in the inner portion. The light wave is efficiently propagated by being reflected by the cladding layer and confined in the core region.
[0007]
[Problems to be solved by the invention]
By the way, in the conventional photonic crystal optical fiber obtained as described above, when the thin quartz rod a and the fine quartz tubes b, b. Residual fusion stress remains in the glass, anisotropic stress is applied to the core region, PMD (polarization mode dispersion) characteristics and the like are deteriorated, and the delicate refractive index distribution control of the inner cladding layer is difficult. is there.
[0008]
Therefore, the present invention has been devised in order to effectively solve such problems, and the purpose thereof is that there is no concentration of anisotropic stress in the core region due to fusion residual stress, and excellent PMD characteristics are obtained. there is provided a novel process for producing fiber-optic with.
[0009]
[Means for Solving the Problems]
In order to solve the above-described problems, the present invention provides a clad layer around a core region, the clad layer being provided around the inner clad layer and the inner clad layer. And forming a glass soot layer around the core glass base material that becomes the core region in the method of manufacturing an optical fiber having an inner cladding layer having a concentric layered bubble region concentric with the core region. diffusion coefficient vitrified by heating the glass soot layer in a gas atmosphere containing a small inert gas than helium, is to form a glass layer having a bubble of the inert gas to be the inner cladding layer at .
[0010]
This eliminates the concentration of anisotropic stress in the core region due to fusion residual stress as in the case of conventional fibers, so that it exhibits excellent PMD characteristics and can easily achieve delicate refractive index distribution control of the inner cladding layer. .
[0011]
In addition, as shown in claim 2 , when the bubbles are formed to have a diameter of 4 μm or less, the above-described effects can be remarkably exhibited.
[0012]
Furthermore, as shown in claim 3, by changing the distribution density of the bubbles in the radial direction, or to have a radial distribution of the equivalent refractive index of the aggregate area of the bubble, as shown in claim 4 If the bubble diameter is changed in the radial direction and the equivalent refractive index of the bubble aggregate region has a radial distribution, the structural dispersion is controlled, and a large positive dispersion fiber and negative dispersion / negative dispersion are controlled. A fiber with a slope is obtained.
[0015]
Further , as shown in claim 5 , the gas atmosphere is formed only of an inert gas whose diffusion coefficient in glass is smaller than that of helium, or as shown in claim 6 , the gas atmosphere is made of helium gas. And a mixture gas of an inert gas whose diffusion coefficient in glass is smaller than that of helium, the above-mentioned effects can be remarkably exhibited. Moreover, as shown in claim 7 , as the inert gas whose diffusion coefficient in the glass is smaller than that of helium, nitrogen gas or argon gas is preferable.
[0016]
DETAILED DESCRIPTION OF THE INVENTION
Next, a preferred embodiment for carrying out the present invention will be described with reference to the accompanying drawings.
[0017]
FIG. 1 shows an embodiment of an optical fiber 1 manufactured by the manufacturing method according to the present invention.
[0018]
As shown in the figure, this optical fiber 1 has an inner cladding layer 3 around a core region 2 located at the axial center, and an outer cladding layer 4 having an outer diameter of about 125 μm around the inner cladding layer 3. Are integrated.
[0019]
The inner cladding layer 3 is formed with a region in which a large number of independent bubbles 5 having a diameter of 4 μm or less are densely gathered, and is present in a layered manner so as to surround the core region 2.
[0020]
In the optical fiber 1 having the structure as described above and manufactured by the manufacturing method of the present invention, the small-diameter quartz rod a and the small-diameter quartz tubes b, b,... Like the conventional photonic crystal optical fiber. Unlike the case where the two are fused and integrated, the concentration of anisotropic stress in the core region due to the fusion residual stress of the thin quartz tube b is eliminated, so that excellent PMD characteristics are exhibited and the inner cladding layer 3 The fine refractive index distribution control can be easily achieved.
[0021]
That is, the optical fiber 1 uses an inert gas having a small diffusion coefficient, such as nitrogen gas or argon gas, as the sintering gas (atmosphere gas) in the sintering process of the inner cladding layer 3 during the preform manufacturing process. The sintered gas is left in the sintered glass layer to form a cellular layer around the core region, and the preform is drawn to an outer diameter of about 125 μm. For this reason, the conventional fusion residual stress does not occur, and a situation such as concentration of anisotropic stress in the core region does not occur. The bubble 5 has a substantially spherical shape during the preform manufacturing process, but becomes a long hole extending in the longitudinal direction by subsequent drawing. Further, by setting the equivalent relative refractive index difference between the core region 2 and the inner cladding layer 3 to 1%, a fiber having a large absolute value of dispersion can be obtained.
[0022]
Here, the diameter of the core region 2 is not particularly limited, but is set to a size of 2 to 10 μm in order to perform single mode transmission. The optimum thickness of the inner cladding layer 3 varies depending on the refractive index profile, dispersion characteristics, etc. of the optical fiber to be obtained, but is generally set in the range of 5 to 30 μm. Further, the absolute condition that the diameter of the bubbles 5 is equal to or less than the layer thickness of the inner cladding layer 3 is 4 μm or less, preferably 1 in consideration of the formation of the bubble density distribution in the inner cladding layer 3. It is desirable that the thickness be ˜3 μm. The optical fiber 1 manufactured by the manufacturing method according to the present invention is not limited to these dimensional conditions.
[0023]
FIG. 2 shows another embodiment of the present invention, in which the refractive index difference distribution is controlled by changing the density of the bubbles 5 constituting the inner cladding layer 3 in the radial direction. That is, the inner cladding layer 3 is divided into three layers on a concentric circle with the core region 2 as the center, and the bubbles 5 are present at a high density in the region on the inner cladding layer 3 side (high density bubble region), and the region outside the region. Is made to have a low density (low density bubble region), and the outer region thereof has a higher density (medium density bubble region). As a result, a multi-step equivalent relative refractive index difference Δn can be generated in the inner cladding layer 3 as shown in the figure, the structural dispersion is controlled, and a large positive dispersion fiber and negative dispersion / negative dispersion slope are provided. Fiber is obtained. Further, even if the distribution density of the bubbles in the inner cladding layer 3 is changed in the radial direction or the outer diameter of the bubbles 5 constituting the inner cladding layer 3 is changed in the radial direction, the distribution in the equivalent refractive index is similarly achieved. It is possible to give it.
[0024]
Here, the outer diameter of the bubbles can be controlled by the following two methods.
One is a method of changing the bulk density of the quartz glass soot layer deposited around the core base material. When soot is vitrified by heating (sintering), the higher the soot bulk density, the smaller the gap through which gas escapes, so the larger bubbles remain, and the lower the soot bulk density, the smaller bubbles remain. The ratio is high.
[0025]
The other is a method of changing the ratio of helium gas and inert gas in the furnace gas atmosphere during sintering. The higher the helium gas ratio, the smaller the bubbles, and the lower the helium gas ratio, the larger the bubbles.
[0026]
Note that the larger the bubbles, the higher the density of the bubbles, and the smaller the bubbles, the lower the density of the bubbles. Therefore, both the bubble diameter and bubble density are controlled by adjusting the bulk density of the quartz glass soot layer and the ratio of helium gas and inert gas in the furnace gas atmosphere during sintering. Can do.
[0027]
Also, if the bubble size and the formation position are controlled so that the interval between the bubbles is adjusted to one half of the wavelength of the signal light to be propagated, even in the optical fiber manufactured by the manufacturing method of the present invention, A band gap structure can be realized.
[0028]
In the embodiment described above, although the core region has been described about the case where the configuration of quartz glass, not limited thereto, the pure silica glass, known impurity for increasing the refractive index (e.g. Either quartz glass to which Ge, Ti, or the like) is added, or quartz glass to which a rare earth element such as Er is added is applicable.
[0029]
Further, the core region may be hollow, and in this case, a hollow quartz tube may be used instead of the above-described core quartz, which is a small-diameter quartz. For this quartz tube, either pure quartz glass, quartz glass to which a known impurity (such as Ge or Ti) for increasing the refractive index is added, or quartz glass to which a rare earth element such as Er is added can be applied. It is.
[0030]
【Example】
Next, examples of the present invention will be described. In Test 1, a bubble aggregate region consisting of one layer of high density bubble region was provided in the inner cladding layer, and in Test 2, three layers of a high density bubble region, a low density bubble region, and a medium density bubble region were formed in the inner cladding layer. A bubble aggregate region composed of layers is provided.
[0031]
<Test 1>
First, the transparent glass base material used as a core area | region was produced by VAD method, and was extended | stretched to the outer diameter of 25 mm.
[0032]
A soot layer that becomes a high-density bubble region was deposited on the outer periphery of the core glass base material by a CVD method to obtain an external base material having an outer diameter of 60 mm. This base material was heat-treated at a temperature of 1600 ° C. in an electric furnace in which the inside of the furnace was in a 100% nitrogen gas atmosphere. The obtained glass base material had a translucent state because the outer diameter was 45 mm and a large number of nitrogen gas bubbles remained in the closed cell assembly region at the outer periphery. The cell density in the high-density cell region was about 0.5.
[0033]
Here, nitrogen gas has a much smaller diffusion coefficient than the helium gas normally used in the heat treatment step for transparent vitrification, and it tends to remain in the glass when soot is vitrified. By setting the atmosphere to 100% nitrogen gas, it is possible to obtain a glass base material containing nitrogen gas bubbles at high density.
[0034]
Next, the obtained glass base material with an inner cladding layer having a high density bubble region is stretched to an outer diameter of 25 mm, a soot layer to be an outer cladding layer is deposited by a CVD method, and an outer base material with an outer diameter of 120 mm is formed. Obtained. This base material was heat-treated at a temperature of 1600 ° C. in an electric furnace in which the inside of the furnace was in a 100% helium gas atmosphere to obtain a glass base material having an outer diameter of 60 mm and a transparent outer cladding layer on the outer periphery.
[0035]
Next, the glass preform was drawn into an optical fiber having an outer diameter of 125 μm by a normal drawing method. The obtained optical fiber had a core diameter of 7 μm, an inner cladding layer thickness of 10 μm, and an outer diameter of bubbles of 1 to 3 μm.
[0036]
<Test 2>
First, the transparent glass base material used as a core area | region was produced by VAD method, and was extended | stretched to the outer diameter of 25 mm.
[0037]
A soot layer that becomes a high-density bubble region was deposited on the outer periphery of the core glass base material by a CVD method to obtain an external base material having an outer diameter of 60 mm. This base material was heat-treated at a temperature of 1600 ° C. in an electric furnace in which the inside of the furnace was in a 100% nitrogen gas atmosphere. Here, the reason for using 100% nitrogen atmosphere gas is as described in Test 1. The glass base material thus obtained was translucent because it had an outer diameter of 45 mm and a large number of nitrogen bubbles remained in the high-density bubble region at the outer periphery. The cell density of this high-density cell region was about 0.5.
[0038]
Next, the glass base material was stretched to an outer diameter of 25 mm, and a soot layer serving as a low-density bubble region was deposited by a CVD method to obtain an external base material having an outer diameter of 60 mm. This base material was heat-treated at a temperature of 1600 ° C. in an electric furnace in an atmosphere containing 20% nitrogen gas and 80% helium gas. Here, the reason why the mixed gas of nitrogen gas and helium gas is used as the atmosphere gas is that the transparency of the glass after the heat treatment becomes higher by mixing the helium gas than in the 100% nitrogen gas atmosphere. This is because a glass base material having a low bubble density can be obtained. The glass base material obtained in this way has an outer diameter of 45 mm, and a slight amount of nitrogen bubbles remain in the low density bubble region on the outer periphery thereof, so that it is more transparent than the high density bubble region. However, it was not completely transparent. The bubble density in the low density bubble region was about 0.3.
[0039]
Next, the obtained glass base material was stretched to an outer diameter of 25 mm, and a soot layer serving as a medium density bubble region was deposited by a CVD method to obtain an external base material having an outer diameter of 60 mm. This base material was heat-treated at a temperature of 1600 ° C. in an electric furnace in an atmosphere of nitrogen gas 50% and helium gas 50%. The glass base material thus obtained had an outer diameter of 45 mm, and the transparency of the medium density bubble region in the outer peripheral portion was in a state having transparency between the high density bubble region and the low density bubble region. The bubble density in the low density bubble region was about 0.4.
[0040]
Next, the obtained glass base material with an inner cladding layer composed of a high density bubble region, a low density bubble region, and a medium density bubble region is stretched to an outer diameter of 25 mm, and a soot layer serving as an outer cladding layer is deposited by a CVD method. Thus, an external base material having an outer diameter of 120 mm was obtained. This base material was heat-treated at a temperature of 1600 ° C. in an electric furnace in a 100% helium gas atmosphere to obtain a glass base material having an outer diameter of 60 mm and a transparent outer cladding layer on the outer periphery. .
[0041]
Next, the glass preform was drawn into an optical fiber having an outer diameter of 125 μm by a normal drawing method. The obtained optical fiber has a core diameter of 4 μm and an inner cladding layer thickness of 12 μm (high density bubble region 4 μm, low density bubble region 5 μm, medium density bubble region 3 μm), and the outer diameter of the bubbles in each region is It was 1-2 μm.
[0042]
【The invention's effect】
In short, according to the present invention, since the cladding layer having an aggregate of bubbles is provided around the core region, concentration of anisotropic stress in the core region due to fusion residual stress as in the conventional photonic crystal optical fiber. And excellent PMD characteristics can be exhibited. As a result, it has low PMD characteristics and exhibits characteristics that cannot be achieved with ordinary optical fibers, such as large effective core area, high refractive index difference, large anomalous dispersion (positive dispersion), negative dispersion / negative dispersion slope fiber, etc. Exhibits excellent effects such as
[Brief description of the drawings]
FIG. 1 is an enlarged sectional view showing an embodiment of an optical fiber manufactured by a manufacturing method according to the present invention.
FIG. 2 is an enlarged sectional view showing another embodiment of an optical fiber manufactured by the manufacturing method according to the present invention .
FIG. 3 is an enlarged perspective view showing the structure of a preform for obtaining a conventional photonic crystal optical fiber.
[Explanation of symbols]
1 optical fiber 2 core region 3 inner cladding layer 4 outer cladding layer 5 bubble

Claims (7)

A clad layer is provided around the core region, and the clad layer is composed of an inner clad layer and an outer clad layer provided around the inner clad layer. The inner clad layer is a collection of layered bubbles concentric with the core region. In the method for manufacturing an optical fiber having a body region, a glass soot layer is formed around a core glass base material to be a core region, and the above is performed in a gas atmosphere containing an inert gas whose diffusion coefficient in glass is smaller than that of helium. A method for producing an optical fiber, wherein the glass soot layer is heated to vitrify to form a glass layer having bubbles of an inert gas serving as the inner cladding layer . The bubble method of manufacturing an optical fiber mounting serial to claim 1, characterized in that formed below the diameter 4 [mu] m. 3. The method of manufacturing an optical fiber according to claim 1, wherein a distribution density of the bubbles is changed in a radial direction so that an equivalent refractive index of the aggregate region of the bubbles has a radial distribution. . The optical fiber manufacturing method according to any one of claims 1 to 3 , wherein the bubble diameter is changed in the radial direction so that the equivalent refractive index of the bubble aggregate region has a radial distribution. Way . The gas atmosphere, an optical fiber manufacturing method according to claims 1 to 4 or, wherein the diffusion coefficient in the glass to form only a small inert gas than helium. The gas atmosphere, an optical fiber manufacturing method according to claims 1 to 5 or the diffusion coefficient of helium gas and the glass is characterized in that it is formed with a mixed gas of small inert gas than helium. The method for producing an optical fiber according to any one of claims 1 to 6 , wherein the inert gas whose diffusion coefficient in the glass is smaller than that of helium is nitrogen gas or argon gas.

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